„Why, oh why, isn‘t there more QUIC use?“ ask people on the QUIC mailing list.
It‘s impossible to answer. Some people probably don‘t care. Some would use it if they could (thanks, OpenSSL, not). For some, they already hit their sweet spot. Some are already using it near 100% (Facebook, controlling servers and clients).
In the curl project, our goal to give you good options to use all protocols, with defaults merely for usability reasons.